Stroke volume (SV) represents the amount of blood pumped out of the ventricle with each beat. This measurement is generally focused on the left ventricle, which circulates oxygenated blood to the entire body. Understanding stroke volume is a foundational concept in assessing cardiovascular health, used by cardiologists and exercise physiologists. It gives direct insight into how effectively the heart muscle is contracting to meet the body’s demands for oxygen and nutrients.
Defining Stroke Volume and Its Components
Stroke volume is typically quantified in milliliters (mL) per beat, and it is derived from two separate measurements taken during the cardiac cycle. The cycle involves two main phases: diastole, the relaxation and filling phase, and systole, the contraction and ejection phase. The calculation of stroke volume directly depends on measuring the volume of blood in the ventricle at the end of each of these phases.
The first component is the End-Diastolic Volume (EDV), the maximum volume of blood the ventricle holds at the end of the filling phase, just before contraction. This represents the total amount of blood available to be ejected during that beat. The second component is the End-Systolic Volume (ESV), the minimum volume of blood remaining in the ventricle after the muscle has finished contracting.
The difference between these two volumes precisely defines the amount of blood that was pushed out into the circulation. A healthy adult male often has an average stroke volume around 70 to 90 mL per beat, though this can vary significantly based on body size and fitness level. By analyzing these volumes, clinicians can determine if the heart is filling correctly and if its muscle is strong enough to empty the ventricle efficiently.
The Mathematical Formula for Stroke Volume
The calculation for stroke volume is a straightforward subtraction that quantifies the difference between the maximum and minimum ventricular blood volumes. The foundational mathematical relationship is expressed as: Stroke Volume (SV) = End-Diastolic Volume (EDV) – End-Systolic Volume (ESV).
For example, if a patient’s ventricle fills with a maximum of 120 mL of blood (EDV) and, after contraction, 50 mL of blood remains (ESV), the resulting stroke volume is 70 mL (120 mL – 50 mL = 70 mL). The value of stroke volume is also used to determine the Ejection Fraction, which is the percentage of the EDV that is ejected with each beat, providing a standardized measure of the heart’s pumping efficiency.
A higher stroke volume generally signals a more efficient heart, as it is moving more blood with each contraction. Conversely, a low stroke volume may indicate the ventricle is not filling completely, or the muscle is failing to contract forcefully enough to empty itself adequately. Monitoring changes in stroke volume over time helps healthcare providers track the progression of heart conditions or the effectiveness of treatments.
How Volumes are Measured in Practice
Clinicians rely on non-invasive imaging techniques to obtain the EDV and ESV values. The most common method is echocardiography, which uses ultrasound waves to create detailed, real-time images of the heart’s structure and function. During an echocardiogram, a technician captures images of the ventricle at its largest point (end-diastole) and its smallest point (end-systole).
Once these two-dimensional images are acquired, specialized software is used to trace the inner boundary of the ventricle. To accurately estimate the three-dimensional volume from a two-dimensional image, clinicians often employ a mathematical method known as the modified Simpson’s rule (or biplane method of disks). This technique assumes the ventricle can be mathematically divided into a series of stacked, short cylindrical slices, or disks, from base to apex.
The software calculates the volume of each theoretical disk by measuring its diameter and height from the traced image data. Summing the volumes of all these disks provides an estimate for the total ventricular volume at both end-diastole and end-systole. This non-invasive, imaging-based approach delivers the EDV and ESV values needed to calculate the stroke volume. Doppler ultrasound, another echocardiography-based technique, can also be used to estimate stroke volume by measuring the velocity of blood flow through the major arteries.